Flutter: There should be exactly one item with [DropdownButton]'s value

Well, since no problem has an exact same solution. I was facing the same issue with my code. Here is How I fixed this.

CODE of my DropdownButton:

DropdownButton(
   items: _salutations
         .map((String item) =>
             DropdownMenuItem<String>(child: Text(item), value: item))
         .toList(),
    onChanged: (String value) {
       setState(() {
         print("previous ${this._salutation}");
         print("selected $value");
         this._salutation = value;
            });
          },
     value: _salutation,
),

The Error

In the code snippet below, I am setting the state for a selection value, which is of type String. Now problem with my code was the default initialization of this selection value. Initially, I was initializing the variable _salutation as:

String _salutation = ""; //Notice the empty String.

This was a mistake!

Initial selection should not be null or empty as the error message correctly mentioned.

'items == null || items.isEmpty || value == null ||

And hence the crash:

Solution
Initialize the value object with some default value. Please note that the value should be the one of the values contained by your collection. If it is not, then expect a crash.

  String _salutation = "Mr."; //This is the selection value. It is also present in my array.
  final _salutations = ["Mr.", "Mrs.", "Master", "Mistress"];//This is the array for dropdown

Might also get this error if trying to set value of dropdown with a class instance;

  var tag1 = Tag();
  var tag2 = Tag();
  print(tag1 == tag2); // prints false, dropwdown computes that value is not present among dropdown options

To solve this override operator ==:

class Tag{
 String name = "tag";

  @override
  bool operator ==(Object other) => other is Tag && other.name == name;

  @override
  int get hashCode => name.hashCode;
}

or use https://pub.dev/packages/equatable lib

class Tag extends Equatable{
 String name = "tag";

  @override
  List<Object> get props => [name];
}

I had the same problem. The solution is simple: you have to be sure that the String that is your default dropdownvalue is contained in the list that you want to use in your dropdownmenu. If you wanted to, let’s say, use a list from an api, you should be sure to know at least one value of that list, so that you could assign it to the variable that is your default dropdownvalue.

Here I want display a list that I obtain from an api. In order to not obtain the error, I set my defaultdropdownvalue with the name ‘Encajes’ that is one of the existing categories that my list contains.

String dropdownValue = "Encajes";

---

    items: categoriesString
    .map<DropdownMenuItem<String>>((String value) {
  return DropdownMenuItem<String>(
    value: value,
    child: Text(value),
  );
}).toList(),